In many telecommunications applications it is advantageous or even necessary that the mutual differences of the transmission delays of protocol data units belonging to a communications stream remain within acceptable limits. Such protocol data units may be e.g. IP (Internet Protocol) packets, ATM (Asynchronous Transfer Mode) cells, Ethernet units, or Frame Relay units. Said communications stream is typically comprised of protocol data units transmitted consecutively in time. For example, in a situation where the communications stream is carrying a voice and/or video signal, variation in the transmission delay will increase the need for buffering the data packets received at the receiving network element such as a router, for instance. Buffering will increase the total delay experienced by said communications stream while it should be as small as possible. In connectionless communications systems, different data protocol units of the communications stream may travel through different routes on their way from the source network element to the destination network element. This means that differing transmission delays experienced by the various data protocol units may result in changing the mutual temporal order between the data protocol units, ie. the receiving order of the data protocol units deviates from the temporal order of transmission of the protocol data units in question. Also, in connection-based communications systems, the communications stream is often directed to travel along two or more parallel routes, e.g. for the reason of exercising load balancing between the different parts of the communications network. Changes in the mutual temporal order of the data protocol units will increase the need for buffering the data protocol units received.
The mutual differences of the transmission delays experienced by the data protocol units should be somehow determined so that it is possible to take corrective action as needed. Said corrective action may consist of, for example, configuring the routing protocol in such a way that parts or areas of the communications network which cause a lot of transmission delay are replaced by other parts or areas of the communications network, and/or the quality classification of parts or areas of the communications network which cause a lot of transmission delay is downgraded in order for a quality classification aware routing protocol to be able to avoid using those parts or areas.
In a prior-art method, a quantity indicating the variation in the transmission delay is calculated based on the transmission and reception times of the protocol data units. To illustrate the method, let us examine two data protocol units PDU1 and PDU2. Protocol data unit PDU1 has been sent from the source network element at moment t_tx1, and protocol data unit PDU2 has been sent at t_tx2. Protocol data unit PDU1 was received in the destination network element at moment t_rx1, and protocol data unit PDU2 was received at t_rx2. The transmission delay experienced by protocol data unit PDU1 is d1=t_rx1−t_tx1, and the transmission delay experienced by protocol data unit PDU2 is d2=t_rx2−t_tx2. The quantity indicating the difference between the two transmission delays is the difference d1−d2=(t_rx1−t_tx1)−(t_rx2−t_tx2)=(t_rx1−t_rx2)−(t_tx1−t_tx2). The latter of the expressions representing the difference of the transmission delays shows that the clocks in the source network element and destination network element need not have a common time, but it suffices that said clocks are mutually frequency-locked, ie. are running at the same rate. In addition to the prerequisite concerning the frequency lock between the clocks the method also requires that the quantity indicating the transmission moment of each protocol data unit is transmitted to the destination network element. In many communications applications, however, these demands are not met.